The present invention relates to a method and an apparatus for measuring absolute rotation with the aid of a light conductive fiber ring interferometer.
Another name for a light conductive fiber ring interferometer is, fiber-optic laser gyro which is described by R. F. Cahill et al: "Phase-nulling fiber-optic laser gyro", Optics Letters, March 1979, Vol. 4, No. 3, p. 93-95, and by R. Ulrich: "Fiber-optic rotation sensing with low drift", Optic Letters, May 1980, Vol. 5, No. 5, p. 173-175.
A prior art light conductive fiber ring interferometer is shown schematically in FIG. 1. The light from a light source Q is fed to a beam dividing arrangement T including gates or parts T1 to T4. The divided light beam exiting via gates T1 and T2 travels into both ends of a light path L formed of a light conductive fiber coil. After passing through light path L, the light beams are combined again in beam dividing arrangement T and the thus produced combined optical output signal at gate T3 is finally received by a photodetector D and converted into an electrical signal which is evaluated to determine the rotation of interest.
In light path L, the optical phase of the light is modulated by a phase modulator Ph by the amount .delta..phi.(t), with the phase modulator Ph being actuated by a periodic, preferably sinusoidal signal at the fundamental frequency f.sub.O, so that the following applies: EQU .delta..phi.(t)=.phi..sub.0 .multidot.sin (2.pi.f.sub.0 t)
On the basis of this modulation, the light recieved by photodetector D is also modulated so that the following sequence develops for its light power P.sub.D : ##EQU1## where the effective phase variation or swing .psi. is defined by, EQU .psi.=2.phi..sub.0 .multidot.sin (.pi.f.sub.0 .tau.),
the light source Q has a power P.sub.Q, C is a constant, .DELTA..phi. is the Sagnac phase shift, J.sub.O, J.sub.1, J.sub.2 are Bessel functions and .tau. is the difference in travel time between the light travel times from gates T1 and T2 to the phase modulator Ph. The Sagnac phase shift .DELTA..phi. is proportional to the rate of rotation to be measured so that such rotation can be determined by measuring the Sagnac phase shift.
The power P.sub.Q of light source Q in the prior art arrangement is unmodulated and constant so that the Sagnac phase shift .DELTA..phi. can be determined, for example, from the optical output signal at gate T3. That is, the signal amplitude A.sub.1 =C.multidot.P.sub.Q .multidot.2.multidot.J.sub.1 (.psi.) sin (2.DELTA..phi.) associated with the fundamental frequency f.sub.O and the signal amplitude A.sub.2 =C.multidot.P.sub.Q .multidot.2J.sub.2 (.psi.) cos (2.DELTA..phi.) associated with the second harmonic 2f.sub.O are initially determined from the optical output signal at gate T3 and thereafter, the quotient ##EQU2## may be formed, for example, with a corresponding electrical circuit. This quotient depends only on the Sagnac phase shift, and on the effective phase variation .psi., which latter quantity can be kept at a constant value with, for example, a regulating arrangement. Therefore, according to equation (2), the Sagnac phase shift 2.DELTA..phi. can be determined with great accuracy from a measurement of the quotient A.sub.1 /A.sub.2.
Optimum modulation conditions result if the fundamental frequency f.sub.O of the phase modulator Ph is selected to conform to f.sub.O =1/(2.tau.). This fundamental frequency f.sub.O, however, has such a high value that evaluation of the electrical output of photodetector D is made more difficult. For example, for a light path L having a light conductive fiber length of 1 km, there results a fundamental frequency f.sub.O =100 kHz. With shorter fiber lengths, the fundamental frequencies are even higher.
For very precise signal processing it is desirable to arrange an electrical analog/digital converter immediately after the photodector D, if possible, or at least through only the intermediate connection of a preamplifier, so that further signal processing can be performed purely digitally until the rate of rotation is obtained. The stated high fundamental frequencies of several hundred kHz can be processed with present-day highly accurate analog/digital converters only in a complicated and therefore expensive manner since conversion times on the order of magnitude of 1 .mu.s are required.